J. Folch-Pi Director of Scientific Research, McLean Hospital, Belmont, Mass., U.S.A. The development of the central nervous system is possibly the most significant aspect of the growth of a mammal from embryo to adulthood. The central nervous system is obviously the main repository not only of the species' inherited functional characteristics but also of the process of individuation. Whatever "engrams" constitute the basis of individual characteristics are laid down mainly in the central nervous system, and especially the brain, during its growth. The chemical aspect of this process IS clearly of great importance and the significance of its study should be self evident. Nevertheless, it is only one aspect of a parel lei series of morphological, physiological, biochemical and psychological events which take place as an integrated process, the final result of which is the transformation of the post-embryonic nervous system into the functioning adult system. It is imperative, therefore, that any study or description of the chemical events during the development of the CNS should be undertaken in ful I awareness of the concomitant morphological, physiological and psychological events. It is only against this multidiscipl inary informational framework that the chemical events during 2 J. FOLCH-PI development can be correctly interpreted and acquire their ful I significance. With this in mind, the introduction to this volume may best serve its purpose by describing briefly the morphological and physiological events that accompany the chemical aspect of development.